Many thousands of vessels form a fleet of marine transportation boats serving companies working in offshore waters throughout the world. The vast majority of these vessels support offshore oil and gas exploration and production activities. Most of these vessels are classed as tugboats, supply boats, or crew boats, and have displacement type hulls. A common problem with such hulls is that it is difficult for men to perform useful work from the decks thereof due to waves which wet the decks and which cause excessive roll and pitch motions of the vessels. Weather in the Gulf of Mexico is occasionally severe enough to hinder or delay operations, but weather in the North Sea is so severe as to have a significant impact on operations from such vessels. Transfer of men and equipment to offshore platforms, handling of anchor buoys, and deployment of men and equipment subsea are examples of operations wherein a safe and stable work platform is needed.
A potential solution to this problem is to build and use larger vessels. This slightly improves the situation, but at a significantly increased cost, and generally it is not considered an economically feasible approach.
Another potential solution is to build a fleet of small to medium sized, column-stabilized, semi-submersible vessels like the larger semi-submersible vessels used for drilling operations. These are stable work platforms having long natural periods of response in the order of 20 to 40 seconds. By contrast, a 200-foot supply boat has natural periods of from 6 to 9 seconds, which is quite close to the period range of highest wave energy. While such semi-submersibles would significantly reduce wave-induced motions and deck wetness, it would be at an excessive cost of retiring the conventional fleet and building a new and more expensive fleet.